The present invention relates to processes for the direct production of hydrogen peroxide from oxygen and hydrogen.
Hydrogen peroxide is typically produced using a two-stage cyclical anthraquinone process. The two-stage anthraquinone process utilizes a working compound (i.e., anthraquinone) dissolved in at least one organic solvent. In the first stage of the anthraquinone process, the working compound is reacted with hydrogen gas in order to reduce the working compound to its hydrogenated form. The hydrogenation of the working compound is accomplished by (1) mixing hydrogen gas with the working compound solution such that at least a portion of the hydrogen dissolves in the working solution and (2) bringing the resulting solution into contact with an appropriate hydrogenation catalyst. In the second stage of the two-stage anthraquinone process, the hydrogenated working compound is oxidized using oxygen, air, or a suitable oxygen containing compound in order to produce hydrogen peroxide and restore the working compound to its original form. The hydrogen peroxide produced in the oxidation step is typically recovered from the working solution by extraction with water.
Several disadvantageous features of the conventional anthraquinone process contribute greatly to the cost of conducting the process. As indicated above, the anthraquinone process requires the use of a working solution composed of a working compound dissolved in at least one organic solvent. During the oxidation stage of the anthraquinone process, hydrogen peroxide product is formed in this working solution at a concentration of only about 1% by weight. Consequently, in order to obtain a desirable overall hydrogen peroxide production rate, an extremely large amount of working solution must be used. All of this working solution must be purified prior to being reused in the anthraquinone process. Additionally, fresh working compound and fresh solvent must be regularly added to the process system in order to make up for working solution losses.
Hydrogen peroxide can also be produced by directly reacting oxygen gas with hydrogen gas using a batch, or semi-batch, autoclave process. In the autoclave process, the hydrogen gas and oxygen gas reactants are reacted in an aqueous liquid medium and in the presence of a hydrogenation catalyst. During the reaction process, the hydrogenation catalyst is suspended in the aqueous reaction medium by agitation.
Unfortunately, direct production of hydrogen peroxide using the autoclave process does not provide a viable commercial alternative to the conventional anthraquinone process. The rate of hydrogen peroxide production realized in the autoclave process is limited by the extremely slow rate at which the hydrogen gas reactant dissolves in the aqueous liquid reaction medium. The rate at which the hydrogen gas reactant dissolves in the liquid reaction medium is particularly slow when a low hydrogen gas to oxygen gas partial pressure ratio (i.e., a nonexplosive hydrogen gas to oxygen gas partial pressure ratio) is maintained in the autoclave system. Given a hydrogen gas partial pressure of 91 psi, an oxygen gas partial pressure of 909 psi, and a moderate degree of agitation, the autoclave process will yield hydrogen peroxide at a rate of only about 0.4 (gmoles H.sub.2 O.sub.2)/(hr, liter of solution) or less. Using an extreme degree of agitation, the autoclave process will yield only from about 0.6 to about 1.2 (gmoles H.sub.2 O.sub.2)/(hr liter of solution). At these low production rates, the total cost of commercially producing a given amount of hydrogen peroxide by the autoclave process would greatly exceed the cost of producing the same amount of hydrogen peroxide using conventional anthraquinone technology.
U.S. Pat. No. 4,336,238 discloses another process wherein hydrogen peroxide is produced by the direct reaction of hydrogen and oxygen. In the process of U.S. Pat. No. 4,336,238, a mixture of hydrogen gas and oxygen gas is contacted with a palladium-on-carbon catalyst in the presence of an acidic, aqueous, liquid reaction medium. The liquid reaction medium contains a large amount (i.e., up to 95% by volume) of an organic solvent. The effective life of the catalyst used in the process of U.S. Pat. No. 4,336,238 is prolonged by continuously removing palladium salts from the liquid reaction medium. These salts are produced during the reaction process as a result of the "solubilization" of the palladium catalyst. The process of U.S. Pat. No. 4,336,238 utilizes a reaction system wherein the liquid reaction medium and the hydrogen gas and oxygen gas reactants are conducted in a plug-flow regime, preferably in an upward direction, through a packed catalyst bed.
The process of U.S. Pat. No. 4,336,238 also fails to provide a viable commercial alternative to the conventional anthraquinone process. The process of U.S. Pat. No. 4,336,238 addresses a common problem experienced in the laboratory when using small scale, fixed bed reaction systems. When using such equipment, a flow system of the type described in U.S. Pat. No. 4,336,238 must typically be employed in order to obtain sufficient liquid holdup and sufficient liquid/catalyst contact for a reaction to occur. However, commercial reaction processes utilizing large scale reaction systems typically are not limited by liquid holdup and/or liquid/catalyst contact. Applicant has determined that, given a large scale fixed bed reaction system and an aqueous liquid reaction medium, the rate at which hydrogen peroxide can be produced by direct reaction of hydrogen gas and oxygen gas is currently limited by the rate at which the hydrogen gas reactant dissolves in the liquid reaction medium.
Several commercial reaction processes utilize trickle-flow reaction systems. Examples of commercial reaction processes utilizing trickle-flow reaction systems include commercial processes for hydrodesulfurization, hydrocracking, hydrotreating, hydrogenation, and oxidation. In a trickle-flow reaction system, a gas/liquid mixture is conducted downwardly through a packed bed in a manner such that the gas and liquid travel through the bed in a trickle-flow regime. In the trickle-flow regime, the liquid component flows, or trickles, through the catalyst in a manner such that the liquid partially wets each catalyst pellet. The gas component, on the other hand, flows downwardly as a continuous phase around the catalyst pellets.